Regulation of Ca2+ transport in brain mitochondria. I. The mechanism of spermine enhancement of Ca2+ uptake and retention

Mechanisms of dual modulatory effects of spermine on the mitochondrial calcium uniporter complex

The mitochondrial Ca 2 + uniporter mediates the crucial cellular process of mitochondrial Ca 2 + uptake, which regulates cell bioenergetics, intracellular Ca 2 + signaling, and cell death initiation. The uniporter contains the pore-forming MCU subunit, an EMRE protein that binds to MCU, and the regulatory MICU1 subunit, which can dimerize with MICU1 or MICU2 and under resting cellular [Ca 2 + ] occludes the MCU pore. It has been known for decades that spermine, which is ubiquitously present in animal cells, can enhance mitochondrial Ca 2 + uptake, but the underlying mechanisms remain unclear. Here, we show that spermine exerts dual modulatory effects on the uniporter. In physiological concentrations of spermine, it enhances uniporter activity by breaking the physical interactions between MCU and the MICU1-containing dimers to allow the uniporter to constitutively take up Ca 2 + even in low [Ca 2 + ] conditions. This potentiation effect does not require MICU2 or the EF-hand motifs in MICU1. When [spermine] rises to millimolar levels, it inhibits the uniporter by targeting the pore region in a MICU-independent manner. The MICU1-dependent spermine potentiation mechanism proposed here, along with our previous finding that cardiac mitochondria have very low MICU1, can explain the puzzling observation in the literature that mitochondria in the heart show no response to spermine.

The Mitochondrial Permeability Transition: Nexus of Aging, Disease and Longevity

The activity of the mitochondrial permeability transition pore, mPTP, a highly regulated multi-component mega-channel, is enhanced in aging and in aging-driven degenerative diseases. mPTP activity accelerates aging by releasing large amounts of cell-damaging reactive oxygen species, Ca²⁺ and NAD⁺. The various pathways that control the channel activity, directly or indirectly, can therefore either inhibit or accelerate aging or retard or enhance the progression of aging-driven degenerative diseases and determine lifespan and healthspan. Autophagy, a catabolic process that removes and digests damaged proteins and organelles, protects the cell against aging and disease. However, the protective effect of autophagy depends on mTORC2/SKG1 inhibition of mPTP. Autophagy is inhibited in aging cells. Mitophagy, a specialized form of autophagy, which retards aging by removing mitochondrial fragments with activated mPTP, is also inhibited in aging cells, and this inhibition leads to increased mPTP activation, which is a major contributor to neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. The increased activity of mPTP in aging turns autophagy/mitophagy into a destructive process leading to cell aging and death. Several drugs and lifestyle modifications that enhance healthspan and lifespan enhance autophagy and inhibit the activation of mPTP. Therefore, elucidating the intricate connections between pathways that activate and inhibit mPTP, in the context of aging and degenerative diseases, could enhance the discovery of new drugs and lifestyle modifications that slow aging and degenerative disease.

The novel fungicide SYP-14288 acts as an uncoupler against Phytophthora capsici

SYP-14288 is a novel fungicide developed by the Shenyang Research Institute of Chemical Industry in China. Although preliminary studies indicate that SYP-14288 is highly effective against 32 important plant pathogens belonging to a range of taxonomic groups, its mode of action remains unknown. In this study, we documented that SYP-14288 has excellent activity against all of the asexual life stages of the plant-pathogenic oomycete Phytophthora capsici, and is especially effective in blocking cyst germination and other life stages that require high energy consumption. In assays designed to determine the fungicide's mode of action, addition of ATP reduced SYP-14288 inhibition of P. capsici, which suggested that SYP-14288 inhibits ATP synthesis of the pathogen. This inference was confirmed in that treatment with SYP-14288 sharply reduced the ATP content in P. capsici. The respiration rate of P. capsici was positively correlated with the concentration of SYP-14288 or of the fungicide fluazinam (an uncoupler of oxidative phosphorylation), but increases in respiration were greater with SYP-14288 than with fluazinam. These results indicate that SYP-14288 is a promising fungicide that functions as an uncoupler of oxidative phosphorylation.

Nutrimetabolomic analysis provides new insights into spermine-induced ileum-system alterations for suckling rats

This study aimed to investigate the effects of spermine supplementation on the ileum metabolism of suckling rats.

Metabolomic strategy for the detection of metabolic effects of spermine supplementation in weaned rats

The purpose of this study is to examine the effects of spermine supplementation on weaned rat metabolism. A metabolomic strategy employing high-resolution (1)H NMR spectroscopy and multivariate data analysis was used to investigate rat biological responses to spermine ingestion. Rats received intragastric administration of either 0.2 or 0.4 μmol/g body weight of spermine or saline for 3 days. Plasma samples taken 48 h after the last spermine ingestion were analyzed. Spermine supplementation significantly increased the plasma levels of 1-methylhistidine, 3-hydroxybutyrate, alanine, glutamate, glycerolphosphocholine, phosphorylcholine, myo-inositol, phenylalanine, lysine, glutamine, trimethylanine, tyrosine, valine, formate, glucose, and lipids. These results suggest that spermine ingestion can alter common systemic metabolic processes, including cell membrane metabolism, lipid metabolism, glucose-alanine cycle metabolism, amino acid metabolism, and gut microbiota metabolism. This study also shows the important role of spermine administration in modulating the metabolism of weaned rats.

The mitochondrial permeability transition pore and its involvement in cell death and in disease pathogenesis

Current research on the mitochondrial permeability transition pore (PTP) and its role in cell death faces a paradox. Initially considered as an in vitro artifact of little pathophysiological relevance, in recent years the PTP has received considerable attention as a potential mechanism for the execution of cell death. The recent successful use of PTP desensitizers in several disease paradigms leaves little doubt about its relevance in pathophysiology; and emerging findings that link the PTP to key cellular signalling pathways are increasing the interest on the pore as a pharmacological target. Yet, recent genetic data have challenged popular views on the molecular nature of the PTP, and called into question many early conclusions about its structure. Here we review basic concepts about PTP structure, function and regulation within the framework of intracellular death signalling, and its role in disease pathogenesis.

The mitochondrial permeability transition from in vitro artifact to disease target

The mitochondrial permeability transition pore is a high conductance channel whose opening leads to an increase of mitochondrial inner membrane permeability to solutes with molecular masses up to approximately 1500 Da. In this review we trace the rise of the permeability transition pore from the status of in vitro artifact to that of effector mechanism of cell death. We then cover recent results based on genetic inactivation of putative permeability transition pore components, and discuss their meaning for our understanding of pore structure. Finally, we discuss evidence indicating that the permeability transition pore plays a role in pathophysiology, with specific emphasis on in vivo models of disease.

Involvement of the mitochondrial calcium uniporter in cardioprotection by ischemic preconditioning

The objective of the present study was to determine whether the mitochondrial calcium uniporter plays a role in the cardioprotection induced by ischemic preconditioning (IPC). Isolated rat hearts were subjected to 30 min of regional ischemia by ligation of the left anterior descending artery followed by 120 min of reperfusion. IPC was achieved by two 5-min periods of global ischemia separated by 5 min of reperfusion. IPC reduced the infarct size and lactate dehydrogenase release in coronary effluent, which was associated with improved recovery of left ventricular contractility. Treatment with ruthenium red (RR, 5 microM), an inhibitor of the uniporter, or with Ru360 (10 microM), a highly specific uniporter inhibitor, provided cardioprotective effects like those of IPC. The cardioprotection induced by IPC was abolished by spermine (20 microM), an activator of the uniporter. Cyclosporin A (CsA, 0.2 microM), an inhibitor of the mitochondrial permeability transition pore, reversed the effects caused by spermine. In mitochondria isolated from untreated hearts, both Ru360 (10 microM) and RR (1 microM) decreased pore opening, while spermine (20 microM) increased pore opening which was blocked by CsA (0.2 microM). In mitochondria from preconditioned hearts, the opening of the pore was inhibited, but this inhibition did not occur in the mitochondria from hearts treated with IPC plus spermine. These results indicate that the mitochondrial calcium uniporter is involved in the cardioprotection conferred by ischemic preconditioning.

Excitotoxic injury to mitochondria isolated from cultured neurons

Neuronal death in response to excitotoxic levels of glutamate is dependent upon mitochondrial Ca2+ accumulation and is associated with a drop in ATP levels and a loss in ionic homeostasis. Yet the mapping of temporal events in mitochondria subsequent to Ca2+ sequestration is incomplete. By isolating mitochondria from primary cultures, we discovered that glutamate treatment of cortical neurons for 10 min caused 44% inhibition of ADP-stimulated respiration, whereas the maximal rate of electron transport (uncoupler-stimulated respiration) was inhibited by approximately 10%. The Ca2+ load in mitochondria from glutamate-treated neurons was estimated to be 167 +/- 19 nmol/mg protein. The glutamate-induced Ca2+ load was less than the maximal Ca2+ uptake capacity of the mitochondria determined in vitro (363 +/- 35 nmol/mg protein). Comparatively, mitochondria isolated from cerebellar granule cells demonstrated a higher Ca2+ uptake capacity (686 +/- 71 nmol/mg protein) than the cortical mitochondria, and the glutamate-induced load of Ca2+ was a smaller percentage of the maximal Ca2+ uptake capacity. Thus, this study indicated that Ca(2+)-induced impairment of mitochondrial ATP production is an early event in the excitotoxic cascade that may contribute to decreased cellular ATP and loss of ionic homeostasis that precede commitment to neuronal death.

Effects of polyamines on mitochondrial Ca2+ transport

Mammalian mitochondria are able to enhance Ca(2+) accumulation in the presence of polyamines by activating the saturable systems of Ca(2+) inward transport and buffering extramitochondrial Ca(2+) concentrations to levels similar to those in the cytosol of resting cells. This effect renders them responsive to regulate free Ca(2+) concentrations in the physioloical range. The mechanism involved is due to a rise in the affinity of the Ca(2+) transport system, induced by polyamines, most probably exhibiting allosteric behaviour. The regulatory site of this mechanism is the so-called S(1) binding site of polyamines, which operates in physiological conditions and is located in the energy well between the two peaks present in the energy profile of mitochondrial spermine transport. Spermine is bidirectionally transported across teh inner membrane by cycling, in which influx and efflux are driven by electrical and pH gradients, respectively. Most probably, polyamine affects the Ca(2+) transport system when it acts from the outside-that is, in the direction of its uniporter channel, in order to reach the S(1) site. Important physiological functions are related to activation of Ca(2+) transport systems by polyamines and their interactions with the S(1) site. These functions include a rise in the metabolic rate for energy supply and modulation of mitochondrial permeability transition induction, with consequent effects on the triggering of the apoptotic pathway.

Ruthenium red-induced transition from ventricular fibrillation to tachycardia in isolated rat hearts:

INTRODUCTION

Ventricular tachycardia (VT) is considered to be the most common precursor of ventricular fibrillation (VF) and sudden cardiac death. However, the mechanisms underlying the transition from VT to VF remain unclear despite more than a century of study. Here, we investigated whether perfusion of the heart with blockers of mitochondrial Ca(2+) uniporter changed the macrodynamics of the heart between VT and VF.

METHODS

The experiments were performed using Langendorff perfused isolated rat hearts in which left ventricular pressure (LVP) and left ventricular cardiomyogram (LVCMG) were measured. Sustained VT or VF was induced by burst pacing of the left ventricular muscles.

RESULTS

During pacing-induced sustained VF, perfusion of the heart with ruthenium red (RR) or Ru 360, blockers of mitochondrial Ca(2+) uniporter, resulted in the reversible conversion of VF to VT. In contrast, during pacing-induced sustained VT, perfusion of the heart with spermine, an activator of mitochondrial Ca(2+) uptake, resulted in the reversible conversion of VT to VF, and the effect was antagonized by cotreatment with RR. In addition, RR-induced conversion of VF to VT was antagonized by cotreatment with S(-)-Bay K8644 (Bay K), an activator of L-type Ca(2+) channels, suggesting that the inactivation of L-type Ca(2+) channels was responsible for the RR-induced effect on the macrodynamics of hearts. In fact, perfusion with verapamil, an antagonist of L-type Ca(2+) channels, during pacing-induced sustained VF, resulted in the conversion of VF to VT.

CONCLUSION

This study demonstrated that perfusion of isolated rat hearts with blockers of Ca(2+) uptake by mitochondria resulted in the reversible conversion of pacing-induced sustained VF to VT, suggesting that changes in mitochondrial Ca(2+) uptake were possibly involved in the transition between VT and VF.

The effect of methylglyoxal-bis(guanylhydrazone) on mitochondrial Ca(2+) fluxes

Methylglyoxal-bis(guanylhydrazone) (MGBG) induces a dose-dependent inhibition of the electrophoretic Ca(2+) uptake by rat liver mitochondria (RLM) without affecting the electrical membrane potential. MGBG is also able to inhibit the electroneutral Ca(2+) release from mitochondria. These effects result in a progressive increase of Ca(2+) level in suspending medium indicating that Ca(2+) uptake is inhibited at higher extent than Ca(2+) efflux. Spermine instead, induces a lowering of external Ca(2+) concentration. This action is reversed by MGBG which again raises the external Ca(2+) concentration then in the absence of spermine, though at a lower extent. The mechanism of MGBG effects and their implications on energy metabolism are discussed.

Ca(2+) efflux in mitochondria from the yeast Endomyces magnusii

Calcium release pathways in Ca(2+)-preloaded mitochondria from the yeast Endomyces magnusii were studied. In the presence of phosphate as a permeant anion, Ca(2+) was released from respiring mitochondria only after massive cation loading at the onset of anaerobiosis. Ca(2+) release was not affected by cyclosporin A, an inhibitor of the mitochondrial permeability transition. Aeration of the mitochondrial suspension inhibited the efflux of Ca(2+) and induced its re-uptake. With acetate as the permeant anion, a spontaneous net Ca(2+) efflux set in after uptake of approximately 150 nmol of Ca(2+)/mg of protein. The rate of this efflux was proportional to the Ca(2+) load and insensitive to aeration, protonophorous uncouplers, and Na(+) ions. Ca(2+) efflux was inhibited by La(3+), Mn(2+), Mg(2+), tetraphenylphosphonium, inorganic phosphate, and nigericin and stimulated by hypotonicity, spermine, and valinomycin in the presence of 4 mm KCl. Atractyloside and t-butyl hydroperoxide were without effect. Ca(2+) efflux was associated with contraction, but not with mitochondrial swelling. We conclude that the permeability transition pore is not involved in Ca(2+) efflux in preloaded E. magnusii mitochondria. The efflux occurs via an Na(+)-independent pathway, in many ways similar to the one in mammalian mitochondria.

Preparation and handling of brain mitochondria useful to study uptake and release of calcium

There is increasing evidence for a critical role of mitochondria in calcium homeostasis and neuronal death in excitotoxicity. In spite of much work during the last two decades, the kinetic parameters of Ca(2+) transport in brain mitochondria remain controversial. Analysis of the literature data suggests that these contradictions can be due to differences in the methodology used to prepare or to incubate brain mitochondria. In the present communication, the whole protocol for preparation of non-synaptic rat forebrain mitochondria is described. This report shows that this preparation is well coupled and essentially free of non-mitochondrial contaminants. The mitochondria obtained are useful to study Ca(2+) uptake and release. Both Na(+)-independent, Na(+)-dependent and spontaneous Ca(2+) release may be studied with this preparation. This system is also useful in studies on the role of mitochondria and other intracellular Ca(2+) stores in disturbance of Ca(2+) homeostasis and delayed cell death under excitotoxic conditions.

Polyamine regulation of plasma membrane phospholipid flip-flop during apoptosis

During apoptosis, phosphatidylserine (PS) is moved from the plasma membrane inner leaflet to the outer leaflet where it triggers recognition and phagocytosis of the apoptotic cell. Although the mechanisms of PS appearance during apoptosis are not well understood, it is thought that declining activity of the aminophospholipid translocase and calcium-mediated, nonspecific flip-flop of phospholipids play a role. As previous studies in the erythrocyte ghost have shown that polyamines can alter flip-flop of phospholipids, we asked whether alterations in cellular polyamines in intact cells undergoing apoptosis would affect PS appearance, either by altering aminophospholipid translocase activity or phospholipid flip-flop. Cells of the human leukemic cell line, HL-60, were incubated with or without the ornithine decarboxylase inhibitor, difluoromethylornithine (DFMO), and induced to undergo apoptosis by ultraviolet irradiation. Whereas DFMO treatment resulted in profound depletion of putrescine and spermidine (but not spermine), it had no effect on caspase activity, DNA fragmentation, or plasma membrane vesiculation, typical characteristics of apoptosis. Notably, DFMO treatment prior to ultraviolet irradiation did not alter the decline in PS inward movement by the aminophospholipid translocase as measured by the uptake of 6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)aminocaproyl] (NBD)-labeled PS detected in the flow cytometer. Conversely, the appearance of endogenous PS in the plasma membrane outer leaflet detected with fluorescein isothiocyanate-labeled annexin V and enhanced phospholipid flip-flop detected by the uptake of 1-palmitoyl-1-[6-[(7-nitro-2-1, 3-benzoxadiazol-4-yl)aminocaproyl]-sn-glycero-3-phosphocholine (NBD-PC) seen during apoptosis were significantly inhibited by prior DFMO treatment. Importantly, replenishment of spermidine, by treatment with exogenous putrescine to bypass the metabolic blockade by DFMO, restored both enhanced phospholipid flip-flop and appearance of PS during apoptosis. Such restoration was seen even in the presence of cycloheximide but was not seen when polyamines were added externally just prior to assay. Taken together, these data show that intracellular polyamines can modulate PS appearance resulting from nonspecific flip-flop of phospholipids across the plasma membrane during apoptosis.

Polyamine modulation of mitochondrial calcium transport. I. Stimulatory and inhibitory effects of aliphatic polyamines, aminoglucosides and other polyamine analogues on mitochondrial calcium uptake

In this study, the regulation of mitochondrial Ca2+ transport by polyamines structurally related to spermine and by analogous polycationic compounds was characterized. Similar to spermine, a number of amino groups containing cationic compounds exerted a dual effect on Ca2+ transport of isolated rat liver mitochondria: a decrease in Ca2+ uptake velocity and an enhancement of Ca2+ accumulation. In contrast to the effects of spermine and other aliphatic polyamines, however, the accumulation-enhancing effect of aminoglucosides, basic polypeptides, and metal-amine complexes turned into an inhibition of Ca2+ accumulation at higher concentrations. Within groups of structurally related compounds, the potency to decrease Ca2+ uptake velocity and to enhance Ca2+ accumulation correlated with the number of cationic charges. The presence of multiple, distributed cationic charges was a necessary, but not sufficient criterion for effects on mitochondrial Ca2+ transport, because cationic polyamines and basic oligopeptides which did not enhance mitochondrial Ca2+ accumulation could be identified. Spermine was not able to antagonize the blocking of Ca2+ uptake by ruthenium red, but rather showed an apparent synergism, which can be explained as a displacement of membrane-bound Ca2+ by spermine. The aminoglucosides, gentamicin and neomycin, but not the inactive polyamine bis(hexamethylene)-triamine, inhibited the binding of spermine to intact mitochondria. Apparently, the binding of spermine, gentamicin, and a number of polyamine analogues to low-affinity binding sites at mitochondria, which have low, but distinct structural requirements and which may correspond to phospholipid headgroups, indirectly influences the activity state of the mitochondrial Ca2+ uniporter. The ability of aminoglucosides to displace spermine from the mitochondria and to inhibit mitochondrial Ca2+ accumulation may contribute to the mitochondrial lesions, which are known to occur early in the course of aminoglucoside-induced nephrotoxicity.

Cyclosporin A inhibits oxidant and calcium stimulated phospholipase D activity in the rat intestinal mitochondria

Mitochondrial swelling and calcium cycling occurs during oxidative stress and can be prevented by cyclosporin A (CysA). Our earlier work has shown that enterocyte mitochondria contains a phospholipase D (PLD) which can be activated by superoxide or calcium. In this study, we have shown that enterocyte mitochondrial PLD activated by these agents can be inhibited by cyclosporin A. This was clearly shown by the absence of phosphatidic acid (PA) formation and phosphatidylethanolamine (PE) degradation. Since this PLD specifically utilizes PE as substrate, PLD activity was also assessed by ethanolamine formation which was inhibited by CysA. CysA also inhibited the cabbage PLD activity as judged by phosphatidylethanol formation. These results suggest that cyclosporin A is an inhibitor of PLD and this may be one of the mechanism by which CysA protects enterocyte mitochondria from oxidative stress.

Beneficial effects of S-adenosyl-L-methionine on blood-brain barrier breakdown and neuronal survival after transient cerebral ischemia in gerbils

We have studied the beneficial effects of S-adenosyl-L-methionine (SAM) tosylate on blood-brain barrier (BBB) breakdown and neuronal survival after transient cerebral ischemia in gerbils. BBB breakdown experiments were performed in pentobarbital anesthetized gerbils subjected to 10 min of bilateral carotid artery occlusion and 6 h of reperfusion. For BBB breakdown measurements, SAM (120 mg/kg, i.p.) was administered to gerbils just after occlusion and thereafter every hour up to 5 h. Fluorometric measurements quantified the blood-brain permeability tracer, Evans blue (EB). SAM treatment significantly reduced the BBB breakdown as indicated by reduced levels of EB fluorescence. Neuronal count experiments were conducted in gerbils subjected to transient ischemia and 7 days of reperfusion. For neuronal count experiments SAM (15-120 mg/kg) was administered at 6 and 12 h after reperfusion, and twice each day thereafter for 7 days. SAM dose dependently protected the hippocampal CA1 neurons assessed by histopathological methods. SAM has a beneficial effect on the outcome of ischemic injury by reducing the BBB breakdown and neuronal death.

Relation between accumulation of phospholipase A2 reaction products and Ca2+ release in isolated liver mitochondria

A Ca(2+)-dependent stimulation of mitochondrial phospholipase A2 is often assumed to play a role in mitochondrial Ca2+ release. We sought to clarify this relation by measuring Ca2+ transport and determining phospholipase A2 reaction products from the same sample of isolated, incubated rat liver mitochondria. When mitochondria had accumulated and spontaneously released again Ca2+, most probably by membrane permeability transition, there was no increase of phospholipase A2 reaction products. However, when the incubation was continued after Ca2+ release, significant increases of the content of lysophosphatidylcholine and unesterified fatty acids could be seen. Quinacrine, an inhibitor of phospholipase A2 activity, prevented Ca2+ release and p-hydroxymercuribenzoic acid, an inhibitor of lysophospholipid reesterification, induced a fast release of Ca2+ from isolated mitochondria. Such effects are usually taken as indirect evidence for a participation of phospholipase A2 in mitochondrial Ca2+ release, but analysis of the mitochondrial lipids revealed that no significant changes of the mass of phospholipase A2 reaction products had occurred. These experiments suggest that the accumulation of phospholipase A2 reaction products in mitochondria is the consequence rather than the cause of the membrane permeability transition. Exogenous phospholipase A2 products, lysophosphatidylcholine and arachidonic acid, induced mitochondrial Ca2+ release after a time lag, which decreased with aging of the mitochondrial preparation. The amount of lysophosphatidylcholine taken up by the mitochondria from the incubation medium during these experiments was measured and compared to the amount of lysophosphatidylcholine produced endogenously by mitochondrial phospholipase A2. From these data it appears likely that the amount of lysophosphatidylcholine generated in the mitochondria after the permeability transition is sufficient to sustain the permeable state. An accumulation of mitochondrially generated phospholipase A2 reaction products after the permeability transition could thus be a decisive factor for the limited reversibility of the membrane permeability transition.

Alcohol inhibits the activation of NAD-linked dehydrogenases by calcium in brain and heart mitochondria

The effect of ethanol on the Ca(2+)-dependent activation of mitochondrial dehydrogenases in rat brain and heart mitochondria was investigated. ADP-stimulated respiration of isolated brain and heart mitochondria (state 3) was stimulated further by submicromolar concentrations of free calcium when respiring on non-saturating concentrations of NAD-linked substrates. The stimulation of oxidative phosphorylation by Ca2+ was correlated with an increase of the mitochondrial matrix free calcium concentration ([Ca2+]m), as measured by fura-2, and with an increased reduction of the mitochondrial NAD(P) pool, indicating an activation of Ca(2+)-dependent dehydrogenases. Sodium inhibited Ca(2+)-dependent stimulation of state 3 respiration and NAD(P) reduction as a result of stimulation of Ca2+ efflux through the Na+/Ca2+ antiporter which reduced the steady-state value of [Ca2+]m. Ethanol stimulated the Na+/Ca2+ antiporter both in brain and heart mitochondria. As a result of this stimulation, ethanol, at pharmacological concentrations (50-300 mM), enhanced the sodium-dependent reduction of [Ca2+]m, and thus attenuated the activation of NAD-linked dehydrogenases and the stimulation of oxidative phosphorylation, by submicromolar concentrations of Ca2+, both in brain and heart mitochondria. This pharmacological effect of ethanol, on brain and heart mitochondria, may be responsible, in part, for the acute and chronic effects of ethanol on brain and heart function and metabolism.

Recent progress on regulation of the mitochondrial permeability transition pore; a cyclosporin-sensitive pore in the inner mitochondrial membrane

The mitochondrial permeability transition pore allows solutes with a m.w. approximately less than 1500 to equilibrate across the inner membrane. A closed pore is favored by cyclosporin A acting at a high-affinity site, which may be the matrix space cylophilin isozyme. Early results obtained with cyclosporin A analogs and metabolites support this hypothesis. Inhibition by cyclosporin does not appear to require inhibition of calcineurin activity; however, it may relate to inhibition of cyclophilin peptide bond isomerase activity. The permeability transition pore is strongly regulated by both the membrane potential (delta psi) and delta pH components of the mitochondrial protonmotive force. A voltage sensor which is influenced by the disulfide/sulhydryl state of vicinal sulfhydryls is proposed to render pore opening sensitive to delta psi. Early results indicate that this sensor is also responsive to membrane surface potential and/or to surface potential gradients. Histidine residues located on the matrix side of the inner membrane render the pore responsive to delta pH. The pore is also regulated by several ions and metabolites which act at sites that are interactive. There are many analogies between the systems which regulate the permeability transition pore and the NMDA receptor channel. These suggest structural similarities and that the permeability transition pore belongs to the family of ligand gated ion channels.

Transport of calcium by mitochondria

The identification of intramitochondrial free calcium ([Ca2+]m) as a primary metabolic mediator [see Hansford (this volume) and Gunter, T. E., Gunter, K. K., Sheu, S.-S., and Gavin, C. E. (1994) Am. J. Physiol. 267, C313-C339, for reviews] has emphasized the importance of understanding the characteristics of those mechanisms that control [Ca2+]m. In this review, we attempt to update the descriptions of the mechanisms that mediate the transport of Ca2+ across the mitochondrial inner membrane, emphasizing the energetics of each mechanism. New concepts within this field are reviewed and some older concepts are discussed more completely than in earlier reviews. The mathematical forms of the membrane potential dependence and concentration dependence of the uniporter are interpolated in such a way as to display the convenience of considering Vmax to be an explicit function of the membrane potential. Recent evidence for a transient rapid conductance state of the uniporter is discussed. New evidence concerning the energetics and stoichiometries of both Na(+)-dependent and Na(+)-independent efflux mechanisms is reviewed. Explicit mathematical expressions are used to describe the energetics of the system and the kinetics of transport via each Ca2+ transport mechanism.

Yeast mitochondrial calcium uptake: regulation by polyamines and magnesium ions

Spermine, spermidine, and magnesium ions modulate the kinetic parameters of the Ca2+ transport system of Endomyces magnusii mitochondria. Mg2+ at concentrations up to 5 mM partially inhibits Ca2+ transport with a half-maximal inhibiting concentration of approximately 0.5 mM. In the presence of 2 mM MgCl2, the S0.5 value of the Ca2+ transport system increases from 220 to 490 microM, which indicates decreased affinity for the system. Spermine and spermidine exert an activating effect, having half-maximal concentrations of 12 and 50 microM, respectively. In the case of spermine, the S0.5 value falls to 50-65 microM, which implies an increase in the transport system affinity for Ca2+. Both Mg2+ and spermine cause a decrease of the Hill coefficient, giving evidence for a smaller degree of cooperativity. Spermine and spermidine enable yeast mitochondria to remove Ca2+ from the media completely. In contrast, Mg2+ lowers the mitochondrial buffer capacity. When both Mg2+ and spermine are present in the medium, their effects on the S0.5 value and the free extramitochondrial Ca2+ concentration are additive. The ability of spermine and Mg2+ to regulate yeast mitochondrial Ca2+ transport is discussed.

Polyamine metabolism in different pathological states of the brain

Biosynthesis of the polyamines spermidine and spermine and their precursor putrescine is controlled by the activity of the two key enzymes ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC). In the adult brain, polyamine synthesis is activated by a variety of physiological and pathological stimuli, resulting most prominently in an increase in ODC activity and putrescine levels. The sharp rise in putrescine levels observed following severe cellular stress is most probably the result of an increase in ODC activity and decrease in SAMDC activity or an activation of the interconversion of spermidine into putrescine via the enzymes spermidine N-acetyltransferase and polyamine oxidase. Spermidine and spermine levels are usually less affected by stress and are reduced in severely injured areas. Changes of polyamine synthesis and metabolism are most pronounced in those pathological conditions that induce cell injury, such as severe metabolic stress, exposure to neurotoxins or seizure. Putrescine levels correlate closely with the density of cell necrosis. Because of the close relationship between the extent of post-stress changes in polyamine metabolism and density of cellular injury, it has been suggested that polyamines play a role in the manifestation of structural defects. Four different mechanisms of polyamine-dependent cell injury are plausible: (1) an overactivation of calcium fluxes and neurotransmitter release in areas with an overshoot in putrescine formation; (2) disturbances of the calcium homeostasis resulting from an impairment of the calcium buffering capacity of mitochondria in regions in which spermine levels are reduced; (3) an overactivation of the NMDA receptor complex caused by a release of polyamines into the extracellular space during ischemia or after ischemia and prolonged recirculation in the tissue surrounding severely damaged areas; (4) an overproduction of hydrogen peroxide resulting from an activation of the interconversion of spermidine into putrescine via the enzymes spermidine N-acetyltransferase and polyamine oxidase. Insofar as a sharp activation of polyamine synthesis is a common response to a variety of physiological and pathological stimuli, studying stress-induced changes in polyamine synthesis and metabolism may help to elucidate the molecular mechanisms involved in the development of cell injury induced by severe stress.

Inhibition by spermine of the inner membrane permeability transition of isolated rat heart mitochondria

The effect of spermine on the permeability transition of the inner mitochondrial membrane of isolated rat heart mitochondria was evaluated. The permeability transition was triggered using a series of agents (t-butyl hydroperoxide, phenylarsine oxide, carboxyatractylate, and elevated Ca2+ and inorganic phosphate concentrations), and was monitored via Ca(2+)-release, mitochondrial swelling and pyridine nucleotide oxidation. By all three criteria, spermine inhibited the transition. A C50 of 0.38 +/- 0.06 (SD) mM was measured for inhibition.

m-iodobenzylguanidine increases the mitochondrial Ca2+ pool in isolated hepatocytes

The incubation of isolated hepatocytes with the inhibitor of protein mono ADP-ribosylation, m-iodobenzylguanidine (MIBG), resulted in an increase in the size of the mitochondrial Ca2+ pool, without alteration of the non-mitochondrial Ca2+ store(s). This increase was abolished when the cytosolic free Ca2+ concentration ([Ca2+]i) was buffered by prior loading of the cells with fluo 3. Elevating [Ca2+]i by releasing the endoplasmic reticular Ca2+ store with 2,5-di-(tert-butyl)-1,4-hydroquinone resulted in a synergistic increase in the magnitude of the mitochondrial Ca2+ pool. A role for protein ADP-ribosylation in the intracellular regulation of mitochondrial Ca2+ homeostasis is suggested.

Dual effect of spermine on mitochondrial Ca2+ transport

1. A dual effect of the polyamine spermine on Ca2+ uptake by isolated rat liver, brain and heart mitochondria could be demonstrated by using a high-resolution system for studying mitochondrial Ca2+ transport. Depending on the experimental situation, spermine had an inhibiting or accelerating effects on mitochondrial Ca(2+)-uptake rate, but invariably increased the mitochondrial Ca2+ accumulation. 2. Both effects were concentration-dependent and clearly discernible on the basis of their different kinetic characteristics. For mitochondria from all three tissues the half-maximally effective concentration for inhibition of the initial rate of Ca2+ uptake was approx. 180 microM, whereas that for the subsequent stimulation of Ca2+ accumulation was approx. 50 microM. 3. Acceleration of the initial uptake rate could be seen when the mitochondria were preloaded with spermine during a 2 min preincubation period and thereafter incubated in a medium without spermine. 4. When such spermine-preloaded mitochondria were incubated in a spermine-containing medium, the increase in Ca(2+)-accumulation capacity was maintained in spite of an unchanged rate of Ca2+ uptake. 5. Mg2+ interacted with the effects of spermine in a differential manner, enhancing the initial inhibition of the rate of mitochondrial Ca2+ uptake and diminishing the subsequent stimulation of mitochondrial Ca2+ accumulation. 6. This dual effect of spermine on mitochondrial Ca2+ transport resolves the apparent paradox that a polycationic compound can act as a stimulator of Ca2+ uptake.

Regulation of cytosolic free calcium concentration by intrasynaptic mitochondria

By the use of digitonin permeabilized presynaptic nerve terminals (synaptosomes), we have found that intrasynaptic mitochondria, when studied "in situ," i.e., surrounded by their cytosolic environment, are able to buffer calcium in a range of calcium concentrations close to those usually present in the cytosol of resting synaptosomes. Adenine nucleotides and polyamines, which are usually lost during isolation of mitochondria, greatly improve the calcium-sequestering activity of mitochondria in permeabilized synaptosomes. The hypothesis that the mitochondria contributes to calcium homeostasis at low resting cytosolic free calcium concentration ([Ca2+]i) in synaptosomes has been tested; it has been found that in fact this is the case. Intrasynaptic mitochondria actively accumulates calcium at [Ca2+]i around 10(-7) M, and this activity is necessary for the regulation of [Ca2+]i. When compared with other membrane-limited calcium pools, it was found that depending on external concentration the calcium pool mobilized from mitochondria is similar or even greater than the IP3- or caffeine-sensitive calcium pools. In summary, the results presented argue in favor of a more prominent role of mitochondria in regulating [Ca2+]i in presynaptic nerve terminals, a role that should be reconsidered for other cellular types in light of the present evidence.

Changes in regional polyamine profiles in rat brains after transient cerebral ischemia (single versus repetitive ischemia): evidence for release of polyamines from injured neurons

Reversible cerebral ischemia (of 5 min, 15 min, or 3-times 5 min) was produced in 14 Mongolian gerbils by occluding both common carotid arteries. After 72 h of recirculation, brains were frozen and processed for measuring regional levels of the polyamines putrescine, spermidine and spermine using HPLC and fluorescent detector. Ischemia induced a marked increase in putrescine levels throughout the brain, most pronounced after 3-times 5 min ischemia (P less than or equal to 0.05 - P less than or equal to 0.001). Spermine levels were significantly reduced, in the hippocampal CA1-subfield after 5 min of ischemia and, in addition, in the striatum and thalamus after 3-times 5 min ischemia. It is suggested that polyamines are released from necrotic neurons and cleared into the blood. Spermine, released from neurons into the extracellular compartment, may bind to the N-methyl-D-aspartate (NMDA) receptor of cells located in close vicinity and may thus render neurons vulnerable to otherwise subtoxic levels of excitotoxins.

The effect of alkanols on Ca2+ transport in brain mitochondria

Ethanol stimulates the Na(+)-dependent Ca2+ efflux in brain mitochondria and inhibits the Na(+)-independent Ca(2+)-efflux. Here, we studied the effects of n-alkanols on the various Ca2+ transport processes in brain mitochondria. Only short-chain alcohols (i.e. methanol, ethanol and propanol) stimulated Na+/Ca2+ exchange. The inhibition of H+/Ca2+ exchange was significant only with ethanol. Short-chain alcohols inhibit while long-chain alcohols activate the cyclosporin-sensitive Ca(2+)-efflux. These data suggest that the mechanism of the alkanols' effects on Na+/Ca2+ exchange, H+/Ca2+ exchange and the cyclosporin sensitive pore are entirely different. Alkanols have no effect on the electrogenic Ca2+ uniporter. Ethanol did not affect the apparent K0.5 for Na+ (7.5 mM) of the Na+/Ca2+ exchange. Similarly, the magnitude of the effect of ethanol did not depend on matrix Ca2+ concentration, suggesting that short-chain alkanols do not stimulate the rate of Na+/Ca2+ exchange by increasing the affinity of the carrier to Ca2+in or Na+out. High concentrations of K+, Mg2+ and Ca2+ enhanced the ethanol effect. It is possible that high surface potential attenuates the effect of ethanol. It is suggested that ethanol stimulation of Na+/Ca2+ exchange depends on the modulation of the surface dielectric constant.

Calcium ions and oxidative cell injury

Exposure of mammalian cells to oxidative stress induced by oxidation-reduction-active quinones and other prooxidants results in depletion of intracellular glutathione, followed by modification of protein thiols and loss of cell viability. Protein thiol modification during oxidative stress is normally associated with impairment of various cell functions, including inhibition of agonist-stimulated phosphoinositide metabolism, disruption of intracellular Ca2+ homeostasis, and perturbation of normal cytoskeletal organization. The latter effect appears to be responsible for formation of the numerous plasma membrane blebs typically seen in cells exposed to cytotoxic concentrations of prooxidants. Following disruption of thiol homeostasis in prooxidant-treated cells, there is impairment of Ca2+ transport and subsequent perturbation of intracellular Ca2+ homeostasis, resulting in a sustained increase in cytosolic Ca2+ concentration. This increase in Ca2+ can cause activation of various Ca(2+)-dependent degradative enzymes (e.g., phospholipases, proteases, endonucleases), which may contribute to cell death. In contrast to the cytotoxic effects of excessive oxidative damage, low levels of oxidative stress can lead to activation of enzymes involved in cell signaling. In particular, the activity of protein kinase C is markedly increased by oxidation-reduction-cycling quinones through a thiol/disulfide exchange mechanism, which may represent a mechanism by which prooxidants can modulate cell growth and differentiation.

Structural requirements of lysophospholipid-regulated mitochondrial Ca2+ transport

Analogues of lysophosphatidylcholine, including PAF (platelet-activating-factor) and HePC (an experimental anticancer drug), were studied for their influence on mitochondrial Ca2+ transport and membrane potential. Lysophospholipids released Ca2+ from mitochondria and reduced the maximal Ca2+ uptake. The structure-activity relations indicate that deprotonated head groups like phosphocholines yield active compounds while partially protonated head groups like phosphoethanolamines are essentially inactive. Structural requirements for the apolar part of the molecules were acyl or alkyl chain lengths of less than 18 carbon atoms at the C1-position of the glycerol backbone and residues of small size and/or low polarity at the C2-position. Choline lysophospholipids, but not ethanolamine lysophospholipids, may therefore induce mitochondrial Ca2+ efflux and become mediators of ischaemic tissue damage where dysregulated phospholipase A2 activity and an impairment of mitochondrial function are supposed to play a crucial role.

FCCP releases Ca2+ from a non-mitochondrial store in an identified Helisoma neuron

The proton ionophore FCCP was evaluated for use as a selective blocker of mitochondrial Ca2+ sequestration in identified Helisoma neurons in vitro. By use of the Ca2+ indicator fura-2, it was found that application of FCCP evoked a gradual increase in cell body [Ca2+]i that reached a level approximately 3-fold higher than baseline after 60 min. Moreover, FCCP released Ca2+ even when added after mitochondrial stores of Ca2+ had previously been emptied by an alternate method. From these and other experiments, it is concluded that FCCP, in addition to its recognized effect on mitochondrial Ca2+ sequestration, also releases Ca2+ from a non-mitochondrial store and is, therefore, unsuitable for use in an intact neuron to selectively inactivate mitochondrial Ca2+ uptake.

The Na(+)-independent Ca2+ efflux system in mitochondria is a Ca2+/2H+ exchange system

The mechanism of the Na(+)-independent Ca2+ efflux system in mitochondria has not been elucidated as yet. With the aid of cyclosporin A, an inhibitor of the Ca2(+)-induced 'pore', and using a variety of inhibitors, uncouplers and ionophores, it is possible to demonstrate, unequivocally, that this process is driven by delta pH. The efflux is not affected by delta psi, thus suggesting an electroneutral Ca2+/2H+ exchange mechanism. Parallel measurements of the rate of Ca2+ efflux and delta pH, as modulated by valinomycin and nigericin, indicate that the rate of efflux is a function of the magnitude of delta pH.

Regulation of Ca2+ transport in brain mitochondria. II. The mechanism of the adenine nucleotides enhancement of Ca2+ uptake and retention

ADP greatly enhances the rate of Ca2+ uptake and retention in Ca2+ loaded mitochondria. Atractyloside, a specific inhibitor of the ADP/ATP translocator, completely inhibits the ADP effect, while bongkrekate, another specific inhibitor of the translocator enhances the effect of ADP. These results indicate that locking the ADP/ATP translocator in the M-state is sufficient to produce the ADP effect. Cyclosporin A, a specific inhibitor of the Ca2(+)-induced membrane permeabilization does not substitute for ADP, indicating that ADP directly affect the rate of electrogenic Ca2+ uptake. The effect of the translocator conformation on the rate of electrogenic Ca2+ uptake is independent of the concentration of Pi and is not caused by changes in membrane potential. However, locking the carrier in the M-state appears to increase the negative surface charge on the matrix face of the inner membrane. This may lead to an enhanced rate of Ca2+ dissociation from the electrogenic carrier at the matrix surface. The rate of Na(+)-independent Ca2+ efflux is only slightly inhibited by locking the carrier in the M-state, presumably due to the same mechanism. In the presence of ADP, Pi inhibits the Na(+)-independent efflux. In the presence of physiological concentrations of spermine, Pi and Mg2+, the rate of Ca2+ uptake, Ca2+ retention and Ca2+ set points depend sharply on ADP concentration at the physiological range of ADP. Thus, changes of cytosolic ADP concentration may lead to change in the rate of Ca2+ uptake by mitochondria and thus modulate the excitation-relaxation cycles of cytoplasmic free calcium.